Skimming centrifuges with vertically or horizontally driven baskets are known. It is also known that, in the case of poorly flowing moist solids, the solids discharge takes place by means of screws instead of by means of chutes.
There are known methods, specifically ultrasonic and infrared measuring methods, which determine the degree of residual moisture of the solids cake in a skimming centrifuge during the dewatering operation by means of contactless measurement. The reflection of the pulses takes place on the surface of the filling, so that only the degree of residual moisture of the surface of the filling is determined.
With ultrasound, measurement is carried out, for example according to DE-A-19 716 128, in such a way that the echo delay of an ultrasound pulse between the ultrasound transmitter, the impinged surface of the filling and the ultrasound receiver is measured and the degree of residual moisture is determined by the evaluation of the quality of the echo of a reflected ultrasound pulse. Measuring errors are caused by differences in the nature of the surface, vapors, mist and dust.
According to laid-open patent application EP-A-891 814, infrared radiation is used for measuring the degree of moisture by the radiation intensity of a light pulse which is reflected from the surface of the filling being measured and the degree of residual moisture being determined by the evaluation of the change in the radiation intensity. Measuring errors are caused here by differences in the nature of the surface and by differences in the color of the filling medium.
It has until now been necessary for a skimming centrifuge to be operated on the basis of empirical values, according to which it is assumed that the solids cake has reached a specific residual moisture after a specific dewatering period, whereupon further working steps can then be carried out, such as for example the skimming discharge of the solids cake. In the case of these empirical values, however, it is necessary to take into account different filtration rates, which are produced for example when there are differently compacted base layers or different suspension characteristics, in particular solids size distribution.
After centrifuging off surplus liquid as a filtrate, the remaining solids cake is dewatered, the degree of residual moisture depending on the dewatering kinetics. The degree of residual moisture varies over the height of the solids cake. The layer of the solids cake that is nearest the basket shell, varying in its height and dependent on the filling medium, is saturated with liquid. The layer of the solids cake lying over that is saturated at the beginning of the dewatering operation and has moisture removed to an extent depending on the dewatering kinematics. At any time during the dewatering, the degree of residual moisture at the surface of the filling is less than or equal to that of the remaining solids cake and is not necessarily always in the same relationship with the degree of residual moisture of the remaining solids cake.
The invention is therefore based on the object of designing a device of the type stated at the beginning in such a way that the degree of residual moisture can be determined in a way in which the product is contacted and the measurement takes place continuously during the dewatering operation and over the entire height of the solids cake or a large proportion of the height of the solids cake, in order to be able to restrict the operating times of the solids-liquid separating device to the amount necessary in each case.